Mutability

An object is considered mutable if if can be changed after it is created, whereas it is immutable if it can not be changed. If you change a mutable object all references to that object will also be changed.

In Python Index, lists, sets and dictionaries are mutable whereas numbers, strings, tuples and frozen sets are immutable.

This is commonly used in interview questions, for example:

a = [0,0]
b = [a,a]
a[1] = 1
print(b) # [ [0,1], [0,1] ]

whereas

a = (0,0)
b = (a,a)
a[1] = 1 # TypeError: 'tuple' object does not support item assignment

though read more about this in Mutability in Python.